Improved contrast between the coloured and transparent states in electrochromic Ni–Al layered double hydroxide films in mixtures of electroactive ions

• Addition of [Ru(bpy)3]2+ darkens the coloured states in Ni–Al-LDH films.
• Addition of [Ru(bpy)3]2+ and [Co(bpy)3]2+ results in a fourfold increase in contrast.
• The improved colour contrast is sustained over multiple potential scans.
• Similar results were obtained for mixtures of [Ru(bpy)3]2+ and [Fe(CN)6]4−.

The simultaneous addition of [Ru(bpy)3]2+ and [Co(bpy)3]2+ to the electrolyte was found to greatly improve the contrast between the bleached and coloured states in electrochromic Ni–Al layered double hydroxide (LDH) films. The relative transmittance of an LDH film recorded in situ during potential scans dropped to 35% on oxidation and returned to 90% on reduction, corresponding to an optical density change (ΔOD) of 0.41, four times larger than the 0.10 obtained in presence of only [Co(bpy)3]2+. This improvement is attributed to mediation of the electrochemical oxidation of the LDH Ni(II) sites by [Ru(bpy)3]3+. The redox potential of [Ru(bpy)3]2+ is higher than that of Ni(II) in the LDHs. The oxidized form of the ruthenium cation can accept electrons from these Ni(II) sites during the anodic scans, resulting in more extensive oxidation of the LDH films. The potential of [Co(bpy)3]2+ is lower than that of the LDH Ni(II) sites. [Co(bpy)3]2+ can donate electrons to the oxidized Ni sites, ensuring the full reduction of the films back to the transparent state during the cathodic scans. In presence of only [Co(bpy)3]2+, oxidation of the film was not as extensive, and the coloured state was not as dark. Similar results were obtained in presence of [Fe(CN)6]4− and [Ru(bpy)3]2+, or of [Fe(CN)6]4− and [Ru(CN)6]4− when the pH of the electrolyte solution was raise to 10. [Fe(CN)6]4− was a more efficient mediator for the reduction of the LDH films than [Co(bpy)3]2+. The colour changes were more reversible with transmittance returning to nearly 100% at the end of each individual scan. However, the coloured state was not as dark. Contrasts in the [Fe(CN)6]4−–[Ru(bpy)3]2+ and [Fe(CN)6]4−–[Ru(CN)6]4− mixtures were lower, with ΔODs of 0.30 and 0.27, respectively.